Waiting for Gore
Now that MoveOn's decided to endorse Barack Obama there's really and truly not much out there worth having in endorsementland except the grand prize of Al Gore. Josh Green steps up to the plate with the breathless speculation you crave:
On the other hand, Obama is now close enough to a big win that Gore’s endorsement could easily put him over the top. Gore is beloved among Democratic primary voters. His staunch denials have been unusually effective in tamping down speculation that he’ll endorse, so an announcement would be earthshaking and guaranteed to dominate the airwaves until the February 5 primaries. Take Tennessee, Gore’s home state, which could wind up making the difference. Democratic polling there is somewhat sparse, especially that done after John Edwards’s withdrawal. But Tennessee looks to be a state in which Clinton currently holds a lead—that is, unless a certain favorite son were to endorse her opponent.
I think that's persuasive. On the other hand, I've never been totally sure where the widespread assumption that Gore prefers Obama has come from. I understand the basis for the idea that there are important Gore-Clinton tensions, but at the same time Gore seems like very much the sort of person who might be sufficiently persuaded by Clinton's arguments about experience, etc. that he just doesn't have strong feelings about the race.